I'm filling up an ArrayList of ArrayList<Integer> type . The insertion is done twice but the result reflects only the second update. Explain

Here is what is going on. list contains a reference to l. So when you clear l, you clear the one you added to list. But the reference is still there to accept new values.

ArrayList<ArrayList<Integer>> list = new ArrayList<>();
int i=0;
ArrayList<Integer> l = new ArrayList<>();
l.add(1);
l.add(4);
list.add(i,l);
i++;

l.clear();    // you just deleted all the contents of list `l` but not the list `l` itself.
l.add(0);
l.add(4);
l.add(3);
l.add(2);    // now list `l` (the reference still in `list` contains 0,4,3,2
list.add(i,l); // and now list contains the same reference yet again so you get

System.out.println(list);

prints

[[0, 4, 3, 2], [0, 4, 3, 2]]

And note that if you do this, list.get(0).set(1,99) will affect both lists because each List l in List list is a reference to the same list.

[[0, 99, 3, 2], [0, 99, 3, 2]]

The answer of WJS is correct.

Just if you wonder how to make it work the way that you expect:

ArrayList<ArrayList<Integer>> list = new ArrayList<>();
   int i=0;
   ArrayList<Integer> l = new ArrayList<>();
   l.add(1);
   l.add(4);
   list.add(i,l);
   i++;

   l = new ArrayList<>(); //Remove l.clear(); and add new instance of the ArrayList
   l.add(0);
   l.add(4);
   l.add(3);
   l.add(2);
   list.add(i,l);

Not to be confusing it is better you to make a new declaration with new name like:

ArrayList<ArrayList<Integer>> list = new ArrayList<>();
int i=0;
ArrayList<Integer> listFirst = new ArrayList<>();
listFirst.add(1);
listFirst.add(4);
list.add(i,listFirst);
i++;

ArrayList<Integer> listSecond = new ArrayList<>();
listSecond.add(0);
listSecond.add(4);
listSecond.add(3);
listSecond.add(2); 
list.add(i,listSecond);

System.out.println(list);